The furnaces commonly used are simple in construction and fired with gas wherever possible, but for testing magnesite and other refractories it is necessary to make use of electrical furnaces. Clay and silica refractories are most conveniently tested in the type of furnace illustrated in Fig. 1 It consists of an iron casing lined with a high-grade refractory and a pedestal upon which the test cones are placed. The lining and containers are best made from a mixture of fused alumina and kaolin in the proportion of 2:1. In order that the required temperatures may be reached it is necessary to use compressed air with the gas, employing a Fletcher burner. The air pressure need not exceed 20 lb. per square inch. Preheating the air by passing it
through a section of 4-in. pipe kept hot by means of an auxiliary burner assists greatly in reaching the desired point. Where natural or city gas is not available vaporized gasoline can be used. In case it is necessary to resort to the use of petroleum the furnace must be made considerably larger to provide space for the combustion of the oil vapor.
The electric furnaces to be used for temperatures much above the melting point of platinum may consist of a tube made from fused magnesia wound with molybdenum or tungsten wire. Such furnaces require that an atmosphere of hydrogen be maintained within to prevent the oxidation of the heating element. For still higher temperatures the Arsem furnace may be employed, consisting essentially of a graphite spiral as the heating element within which a tube of fused magnesia serves as the container of the specimens to be tested. The furnace is enclosed in a gastight jacket and a vacuum is maintained by means of a pump. The specimen is observed through a glass looking down into the tube and the softening temperature determined by means of the Holborn-Kurlbaum optical pyrometer. Usually there is no room for placing the control cones.
Electrical furnac.es of the types just mentioned require considerably more attention and care than they can receive in works laboratories and it is advisable to employ apparatus of a simpler type. A furnace of this kind is shown in Fig. 2, consisting essentially of a tube, A, pressed from granulated, fused magnesia mixed with linseed oil. Somewhat impure magnesia is to be preferred, containing from 5 to 10 per cent of ferric oxide, silica and alumina. Around this tube open at both ends, in the space B, granular electrode carbon, varying in size between grains passing the 12- and 20-mesh sieves, is placed, the cross-section being constricted at the middle of the tube. This increases the resistance throughout the section and causes the temperature to be correspondingly higher. The electrodes E\ and E% are rings of wrought iron connecting to the leads. The cones are placed upon a refractory cylinder which is fastened to a disc arranged to move up or down. The carbon is confined by rings of refractory material made preferably from a mixture of alundum and kaolin. There are on the market also well-constructed furnaces such as the Hoskins and Northrup types which are adapted for this work.
It is common practice to state the softening point of a material by saying that it corresponds to that of a certain cone number, or we may say that a clay softens between, say, cones No. 31 and 32. The practice of referring to the published softening temperatures rather than the cone numbers is not to be recommended, especially as the values usually given are derived from old sources and have been shown to be
W/J////M//W//////////////////////////7I/r FIG. 2.—Electrical furnace.